A team of more than 360 scientists around the world have spent the past decade surveying 25 regions, from the Antarctic through the temperate and tropical seas to the Arctic to count the different types of plants and animals.

The results show that around a fifth of the world's marine species are crustaceans such as crabs, lobsters, krill and barnacles. Add in molluscs (squid and octopus) and fish (including sharks) and that accounts for up to half of the number of species in the world's seas. The charismatic species often used in conservation campaigning – whales, sea lions, turtles and sea birds – account for less than 2% of the species in the world's oceans.

The surveys have also highlighted major areas of concern for conservationists. "In every region, they've got the same story of a major collapse of what were usually very abundant fish stocks or crabs or crustaceans that are now only 5-10% of what they used to be," said Mark Costello of the Leigh Marine Laboratory, University of Auckland in New Zealand. "These are largely due to over-harvesting and poor management of those fisheries. That's probably the biggest and most consistent threat to marine biodiversity around the world."